GB2242936A - Oil sealed rotary compressors - Google Patents

Abstract

A rotary oil sealed compressor comprises an outer casing (2) containing a stator (4) within which is a rotor (6). The lower portion of the casing (2) constitutes an oil sump which, in use, contains oil up to a nominal level (22). Oil injectors (24) are arranged to inject oil from the sump into the stator. The compressed air outlet (16) from the stator (4) is arranged to discharge compressed air downwardly at a point above the nominal level (22) whereby, in use, the compressed air leaving the stator impinges against the surface of the oil in the sump and the major proportion of the oil droplets entrained in the compressed air coalesce against the surface of the oil. The casing (2) communicates with the compressor outlet via a coalescing element (30) through which the compressed air flows and the remaining entrained oil droplets are substantially removed from the compressed air. <IMAGE>

Description

OIL SEALED ROTARY COMPRESSORS The present invention relates to oil sealed rotary compressors, e.g. of sliding vane eccentric rotor type, and is concerned with the separation of the oil from the compressed air.

Oil sealed compressors are those compressors in which oil is injected into the compression space defined by the stator and is subsequently discharged from the stator with the compressed air and must be removed from the compressed air and then recycled. The purpose of the oil is firstly to lubricate the compression elements, secondly to enhance the seal between the cooperating compression elements or between the rotor and stator and thirdly to act as a thermal carrier for removal of the heat produced by the compression process which is effected. The oil becomes entrained in the compressed air and is discharged with it from the stator in the form of fine droplets.It is then essential that substantially all the oil be separated from the compressed air in order to avoid the compressed air leaving the compressor being contaminated with oil and also to avoid the necessity and expense of frequently topping up the oil supply of the compressor.

It is not practical to remove all the oil from the compressed air in a single separation stage and it is therefore usual to provide successive primary and secondary oil separators. The primary separator is normally situated immediately adjacent the outlet from the stator and may constitute an impingement shield, as in the compressor disclosed in GB-A-1318884, against which the compressed air impinges at high velocity, whereby substantially all the oil droplets coalesce on the surface of the impingement shield and then drip down into the oil sump defined by an outer casing containing the rotor/stator unit. Alternatively or additionally, the primary separator may include a labyrinthine pathway along which the compressed air flows and against the walls of which the oil droplets coalesce and then drip down to the sump.In both cases the compressed air then flows through the secondary separator which includes one or more filters or coalescing elements which coalesce substantially all the remaining oil droplets which are then returned, e.g. to the compressor inlet or to the interior of the rotor/stator unit, under the action of the pressure differential between those points and the secondary separator.

Whilst the efficiency of the primary oil separator is usually of the order of 99.9%, it is desirable that this efficiency be as high as possible in order that the risk of the secondary separator becoming clogged with oil is minimised and that the frequency with which the secondary separation filter or coalescing element must be replaced is minimised also. The efficiency with which the secondary separation filter or coalescing element may be constructed to operate is also an inverse function of the maximum oil load to which it is likely to be subjected.However, it is found that the vigour with which the compressed air flow impinges against the primary separation surface is such that a proportion of the oil which is coalesced is subsequently fragmented or atomised by the compressed air jet and that the fine droplets which are thus formed tend not to recoalesce but to be carried over to the secondary separator. Furthermore, the known types of primary separator are relatively bulky and add not insignificantly to the weight and cost of the compressor and also result in the outer casing of the compressor within which the rotor/stator unit is accommodated being rather larger than would otherwise be the case.

It is therefore the object of the present invention to provide a rotary oil sealed compressor with primary and secondary oil separation stages in which the primary oil separation stage is smaller, lighter and cheaper and also more efficient than those which have previously been used.

According to the present invention a rotary oil sealed compressor comprises an outer casing containing a stator within which are one or more rotors, an oil sump defined by the lower portion of the casing which, in use, contains oil up to a nominal level and means for injecting oil from the sump into the stator, the compressed air outlet from the stator being arranged to discharge compressed air downwardly at a point above the said nominal level whereby, in use, the compressed air leaving the stator impinges against the surface of the oil in the sump and the major proportion of the oil droplets entrained in the compressed air coalesce against the surface of the oil, the casing communicating with the compressor outlet via a filter or coalescing element through which the compressed air flows and the remaining entrained oil droplets are substantially removed from the compressed air.

Thus the compressor in accordance with the present invention has primary and secondary oil separation stages, as is conventional, but the primary separation surface against which the compressed air leaving the stator impinges is afforded not by a separate impingement shield or labyrinthine passage, as previously, but simply by the surface of the oil in the sump. This results not only in a decrease in the size, weight and expense of the compressor but also, surprisingly, in an increase in separation efficiency of the primary separation stage. The powerful flow of compressed air leaving the stator outlet forms a meniscus or arcuate-shaped depression in the oil surface against which the entrained oil droplets impinge and are for the most part coalesced.Due to the fact that the oil against which the compressed air jet impinges always presents a substantially smooth surface it is found that the fragmentation and reentrainment of coalesced oil is substantially reduced.

The air which impinges against the surface of the oil is then obliged abruptly to reverse its direction of flow, this abrupt change in direction results in a very substantial centrifugal force being exerted on these remaining oil droplets and this results in the major proportion of them coalescing with a portion of the surface of the recess formed in the oil surface which is not directly opposed to the stator outlet. The combination of these two effects results in a primary separation efficiency in excess of 99.9%. The compressed air together with any remaining entrained oil droplets then flows through a secondary oil separator which includes a filter or coalescing element in which the remainder of the oil droplets are coalesced.The clean compressed air then flows out of the compressor outlet and the oil removed by the secondary separator is then returned to the compressor inlet or to the interior of the stator under the action of a pressure differential, in the normal manner.

It is usual for the stator to be provided with a number--of outlet-openings spaced along its length and it is preferred that these openings communicate with a common outlet manifold which in turn communicates with one or more outlet pipes whose downstream end is directed downwardly and terminates at a point adjacent to and shortly above the normal oil level.

The downstream end of the outlet pipe should not be situated too great a distance from the surface of the oil in the sump since otherwise the oil droplets might not impinge against the oil surface with sufficient force to coalesce them and the meniscus or depression in the oil surface which enhances the separation efficiency would not be formed. Similarly, the downstream end of the outlet pipe should not be situated too close to the oil surface since otherwise the vigour with which the compressed air impinges against it might be sufficient to fragment and entrain oil from the sump into the compressed air which flows towards the secondary separator. It has been found that the downstream end of the outlet pipe should preferably be between 0.5D and 1.5D, where D is the diameter of the outlet pipe.

The compressed air leaving the stator is in a state of very considerable turbulence but it is desirable that the airflow leaving the outlet pipe should be as coherent as possible since this enhances the efficiency with which the oil droplets are coalesced against the oil surface. It is therefore desirable that the outlet pipe be relatively long and have no abrupt changes in direction and this may be achieved if the outlet openings are formed in the upper portion of the stator on one side of the axis of the rotor and the outlet pipe extends above the stator and passes through the plane containing the axis of the rotor.

Further features and details of the invention will be apparent from the following description of one specific compressor in accordance with the invention which is given by way of example with reference to the single accompanying drawing which is a transverse section view through an oil sealed compressor of sliding vane eccentric rotor type.

The compressor includes an outer casing 2 within which is a rotor/stator unit comprising a stator 4 within which an eccentric rotor 6 is rotatably mounted.

The rotor and stator together define a crescent-shaped compression space and the rotor is formed with a plurality, in this case six, of radial slots, each of which contains a sliding vane 8. The vanes 8 are in sliding contact with the wall of the stator and divide the compression space into individual compression cells. Formed in the stator 4 and spaced apart along its length is a plurality of outlet openings 10 which communicate with a common outlet manifold 12.

Communicating with the manifold 12 is an elbow 14 connected to which in turn is a curved outlet pipe 16 which passes over the top of the stator and terminates at a position shortly above the axis of the rotor, as will be described in more detail below.

The lower portion of the casing 2 constitutes an oil sump which may be filled with oil through a filling opening 18 which is normally closed by an oil plug 20.

The sump is filled with oil through the opening 18 up to a normal or nominal level 22, which in this case is at the same height as the rotor axis. Communicating with the oil sump are oil passageways (not shown) which communicate with oil injectors 24 (shown only diagrammatically) arranged to inject oil into the compression space.

Integral with the outer casing 2 is a secondary separation casing 26 whose interior communicates via a passage 28 with the interior of the casing 2 and accommodates a tubular coalescing element 30.

In use, the rotor 6 is rotated in the anticlockwise direction as seen in the drawing and the vanes 8 are maintained in contact with the surface of the stator by centrifugal force. The volume of each compression cell thus progressively increases and then decreases and whilst it is increasing it is in communication with the compressor inlet (not shown), whereby air is drawn into the compression cell. Oil is injected into it through the oil injectors 24 under the action of the pressure differential between the oil sump, which is at the compressor outlet pressure, and the compression cell. As each compression cell approaches its minimum volume it comes into communication with the outlet openings 10 and the compressed air within it is discharged into the manifold 12 and then flows through the elbow 14 and pipe 16.The compressed air together with the entrained oil droplets in it is discharged downwardly from the downstream end of the outlet pipe 16 against the surface 22 of the oil in the sump. The downstream end of the outlet pipe 16 is spaced from the normal surface level 22 of the oil by a distance approximately equal to its own diameter and the force of the compressed air forms a depression, whose depth will depend on the air throughput of the compressor, in the surface of the oil. Due to the considerable length of the outlet pipe 16 and the fact that it is of gentle and substantially constant curvature, the air leaving the outlet pipe has a generally coherent flow pattern and the entrained oil droplets in it impinge against the bottom of the depression in the oil surface and most of them coalesce against it.The compressed air rapidly changes its direction from flowing downwards to flowing generally upwards and any oil droplets which have not coalesced continue to flow with it. These droplets are therefore subjected to a very considerable centrifugal force at reversal which results in a further proportion of the oil droplets coalescing against a portion of the surface of the depression in the oil. The compressed air together with perhaps only 0.1% of the originally entrained oil droplets then flows upwardly and transversely across the interior of the casing 2 and passes out through the passage 28 into the interior of the secondary separator housing 26. It then flows through the wall of the coalescing element 30 and passes out through the compressor outlet (not shown).

Oil coalesced by the element 30 accumulates in the bottom of the housing 26 and in the bottom of the coalescing element 30 and from there is returned to the compressor inlet or to the compression space via an oil return line (not shown) under the action of the pressure differential.

Due to the effective elimination of a complex oil separation system including a primary oil separation stage, the function of which is fulfilled by the outlet pipe 16 and the oil in the sump, the compressor is lighter and cheaper than would otherwise be the case and the outer casing 2 is also somewhat smaller than would otherwise be the case.

Whilst the invention has been specifically described in relation to a compressor of sliding vane eccentric rotor type, it will be appreciated that it is applicable to any oil sealed compressor, that is to say a compressor in which oil is injected into the compression space and must subsequently be removed from the compressed air, and is thus applicable also to compressors of e.g. screw type.

Claims (6)

1. A rotary oil sealed compressor comprising an outer casing containing a stator within which are one or more rotors, an oil sump defined by the lower portion of the casing which, in use, contains oil up to a nominal level and means for injecting oil from the sump into the stator, the compressed air outlet from the stator being arranged to discharge compressed air downwardly at a point above the said nominal level whereby, in use, the compressed air leaving the stator impinges against the surface of the oil in the sump and the major proportion of the oil droplets entrained in the compressed air coalesce against the surface of the oil, the casing communicating with the compressor outlet via a filter or coalescing element through which the compressed air flows and the remaining entrained oil droplets are substantially removed from the compressed air

2.A compressor as claimed in Claim 1 in which there are a plurality of outlet openings in the stator which communicate with a common outlet manifold which in turn communicates with one or more outlet pipes which terminate at a point above the said nominal level.

3. A compressor as claimed in Claim 2 in which the downstream end of the outlet pipe is situated above the nominal oil level by a distance which is between 0.5D and 1.5D, where D is the diameter of the outlet pipe.

4. A compressor as claimed in Claim 2 or 3 in which the outlet openings are formed in the upper portion of the stator on one side of the axis of the rotor and the outlet pipe extends above the stator and passes through the plane containing the axis of the rotor.

5. A compressor as claimed in any one of the preceding claims which is of sliding vane eccentric rotor type.

6. A compressor substantially as specifically herein described with reference to the accompanying drawing.